In the fabrication of microelectronic devices and micromechanical devices several layers of material are deposited and worked on a single substrate to produce a large number of individual devices. After forming the devices in and/or on the substrate, the substrate is cut into a plurality of pieces to separate the individual devices from each other. In the process of forming these devices on the substrate, layers of photoresist are deposited and worked (i.e., patterned, developed, etched, and so forth) to form the features of the devices in and/or on the substrate.
One photoresist deposition technology used in the fabrication of microelectronic devices is a spin-on resist processes in which the resist is deposited onto a spinning substrate. The resist spreads across the substrate under the influence of centrifugal force. For the creation of some devices, however, spin-on resist processes are problematic. For example, one problem of spin-on techniques is that excess photoresist may be deposited on certain portions of the substrate and/or an insufficient amount of photoresist may be deposited on other portions. Another problem of spin-on techniques is that the layer of photoresist may not conform to the topography of the features on a workpiece. This problem can be a significant concern if the resist is to cover large step heights or small features. As the devices become more complicated, the constraints of forming uniform, conformal layers of photoresist may exceed the capabilities of spin-on processes. Further, some processes require the application of thick layers of resist, but spin-on processes may not be able to deposit the resist to the required thickness. Therefore, spin-on techniques may not provide adequate results for depositing resist onto microelectronic devices in many situations.
A different photoresist deposition technique used in the manufacture of printed circuit boards is EPR deposition. The electrolytic deposition of EPR in the printed circuit board industry is normally carried out through a “rack-and-tank” type system in which the workpiece is hung vertically and both sides of the workpiece are submerged in an electrolytic bath containing the emulsion. FIG. 1 schematically illustrates a conventional rack-and-tank type system 1 for electrolytically depositing EPR onto printed circuit boards. The rack-and-tank type system 1 includes a plurality of tanks 2a–2e that include various solutions and/or emulsions 3a–3e. For example, the system 1 can include a preclean tank 2a having a preclean solution 3a, a rinse tank 2b having a rinse solution 3b, an electrolytic tank 2c having an EPR emulsion 3c, a permeate tank 2d having a permeate solution 3d, and another rinse tank 2e having a rinse solution 3e. The electrolytic tank 2c includes electrodes 4a and 4b. The system 1 also includes a rack 5 having a plurality of hangers 6 that suspend individual workpieces 7 in a vertical orientation. The hangers 6 also electrically contact conductive layers on the workpieces 7 to which the EPR is plated. In operation, the rack 5 moves the workpieces 7 through the tanks 2a–2e to preclean, rinse, deposit EPR, dip in a permeate solution, and then rinse. When the workpieces 7 are submerged in the EPR tank 2c, a voltage differential is applied to the electrodes 4a–b and the workpiece 7 causing the resist material to coat the workpiece.
Electrolytically depositing EPR has several benefits that may be useful in applications for microelectronic workpieces. First, electrolytic EPR can deposit a uniformly thick layer of resist across a workpiece. Second, electrolytic EPR deposition can form layers that conform to highly topographical surfaces. Third, electrolytic EPR deposition can form thick layers of resist with good uniformity.
The conventional rack-and-tank type systems used for printed circuit boards, however, are not suitable for integration with other automated microfabrication tools because station-to-station contamination may be problematic. For example, rack-and-tank deposition systems are relatively messy because they fully submerge both sides of the workpiece in the EPR bath. This completely coats the workpiece with emulsion, and thus there is no clean area on the workpiece for robotic transfer units to transfer the workpieces among processing stations. Furthermore, rack-and-tank type systems are prone to forming or entraining bubbles in the EPR bath during deposition. In the case of the electrophoretic deposition of a photoresist, bubbles can migrate to the workpiece and result in defects in the resist layer called “pinholes.” The pinhole defects are generally 10 μm–50 μm in diameter. Areas of the workpiece having pinhole defects may not be patterned with small microelectronic structures and are thus potentially wasted areas of the workpiece. For example, even if defects occur in only 10% of the chip sites at each mask step during photolithography, less than 50% of the chips will be functional after a seven mask process is completed. Conventional rack-and-tank type EPR equipment mitigates pinholes by holding the workpiece vertically and vibrating the workpiece in the EPR bath. The vibration energy used for printed circuit boards, however, typically exceeds the force that can be safely applied to semiconductor workpieces (i.e., semiconductor wafers) and other types of delicate microelectronic workpieces.
Electrolytic EPR deposition equipment for printed circuit boards has been proposed for use in coating microelectronic workpieces in the production of microelectronic devices, such as semiconductor devices and micromachines. However, it has been generally rejected by the microfabrication industry. As the present inventors have recognized, the printed circuit board EPR deposition equipment is not suited to the close tolerance work, cleanliness, and throughput requirements typically expected in the microfabrication industry. For example, full submersion of the workpiece during EPR deposition renders conventional EPR deposition methods impractical for integration into automated, multi-stage processing tools for microelectronic device processing because the EPR will foul and contaminate the single-wafer type robotic handling equipment. Additionally, the equipment for EPR deposition is prone to the formation of bubbles during the deposition process, and the conventional system of vibrating the workpiece may break several workpieces. Other problems of using EPR deposition systems designed for processing printed circuit boards in applications for fabricating microdevices on semiconductor wafers or other types of workpieces include: (a) contamination of other processing stations; (b) additional steps for removing edge beads from workpieces; and (c) insufficient electrical contact with the workpieces.